In the optical communication field or the optical measurement field, an optical waveguide device is frequently used for an optical modulator. FIG. 1 is an example of an optical waveguide device disclosed in Patent Literature No. 1. In FIG. 1, an optical waveguide 2 is formed on a substrate 1 having an electro-optic effect, such as lithium niobate. The optical waveguide 2 is a Mach-Zehnder waveguide provided with branched waveguides (211, 212). A modulation electrode 3 for applying an electric field corresponding to a modulation signal to the optical waveguide and a bias electrode 4 for applying an electric field corresponding to a DC bias are formed along the optical waveguide.
The bias electrode is formed side by side with the modulation electrode along a length direction of the waveguide, and therefore, if the modulation electrode is made long in order to reduce a half-wavelength voltage (Vπ) which is required for a modulation operation, the length of the bias electrode becomes short. Therefore, the bias electrode has to have an electrode configuration in which the operational effect on the optical waveguide is higher.
The modulation electrode 3 is configured of signal electrodes (311, 312) and ground electrodes (321 to 323). The bias electrode configures a so-called “differential bias electrode” which is provided with a pair of electrodes (41, 42), and in which direct current (DC) bias voltages (V1, V2) are applied to the respective electrodes.
A plurality of electrode portions (411 to 413, 421 to 423) are formed along the optical waveguide in the respective electrodes configuring the bias electrode. For example, the electrode 41 has a comb-shaped structure in which the plurality of electrode portions (411 to 413) branched from one electric line are disposed in parallel. The electrode 42 also has a comb-shaped structure in a similar way.
FIG. 2 shows an example of another optical waveguide device, and the optical waveguide formed on the substrate 1 is a nested optical waveguide in which a plurality of Mach-Zehnder waveguides are disposed in a nested manner. In the optical waveguide device of FIG. 2, the bias electrode 4 is configured of two pairs of electrodes (43 and 44, 45 and 46), and at a portion where an electric field corresponding to a DC bias is applied to the optical waveguide, a plurality of electrode portions are disposed in a comb shape. The interval between the electrodes is narrow in order to enhance the operational effect on the optical waveguide.
As a modulation form which is applied to the optical waveguide device in order to realize high-frequency and large-capacity optical fiber communication, in addition to conventional intensity modulation (On-Off keying) or the like, multilevel modulation such as Quadrature Phase Shift Keying (QPSK) using phase modulation or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK), or a transmission format with polarization multiplexing incorporated into the multilevel modulation is the main stream. For this reason, the number of modulation portions which are incorporated in one substrate also increases, and accordingly, the number of locations to which a DC bias is applied also increases. As a result, a large number of electrodes each having a comb-shaped structure as shown in FIG. 1 or FIG. 2 are disposed on the substrate.
If two electrodes each having a comb-shaped structure are disposed so as to mesh with each other, the differential bias electrode functions as an antenna or a slot line due to capacitive coupling, electric field coupling, or the like at a portion having a comb-shaped structure. For this reason, in a case of functioning as an antenna, extraneous electromagnetic waves (microwaves) can be easily picked up as noise, and in a case of functioning as a slot line, electric noise propagating through the bias electrode can be easily picked up. As a result, a phenomenon occurs in which the DC bias which is applied to the optical waveguide is destabilized.
Further, if noise which is picked up increases, an electric discharge also occurs between the electrodes each having a comb-shaped structure, and thus there is also a concern that the electrode itself may be damaged.